Therefore, columns with an internal diameter ranging from 0.25 mm to 0.53 mm and a vaporization chamber (liner) of an adequate volume, which allows the introduction of samples, in splitless mode, ranging from 1-3 μl, are normally used in gas chromatography. In these cases, without modifying the column or the vaporization chamber, the present invention allows the injection in splitless mode of samples greater than 5 μl and up to 50 μl and over.
Moreover, the use of “narrow bore” columns, with an internal diameter below 0.25 mm (typically from 0.10 to 0.18 mm), is becoming increasingly frequent. However, these columns, which have the advantage of a greater efficiency per unit of length, require the sample to be transferred from the vaporization chamber to the column in extremely brief times. This condition is exceedingly difficult to satisfy owing to the low flow rate at which these columns must work. This makes conventional splitless injection almost impossible and it is usually necessary to use a split injection, where only a small fraction of the sample is transferred to the column (usually only 1/50- 1/100 of 1 μl injected). The present invention makes it possible to transfer the sample in splitless mode also in chromatography with “narrow bore” columns. This aspect extends the range of application of the present invention, summed up here: 1) splitless injections of samples greater than 5 μl and up to 50 μl and over with traditional columns and 2) splitless injection of samples up to 1-5 μl and over with “narrow bore” columns.
Attempts have been made, in traditional chromatography, to increase the injectable volume of the samples, as current trends are aimed at an increase in the volumes to be injected, in order to be able to detect even very small quantities of the compound to be analysed and in a more straightforward manner; for this purpose techniques have been developed for discharging to the outside, without introducing them onto the gas chromatographic column, the majority of the solvent vapours which are formed first and are eliminated from the vaporization injection chamber. However, this often causes loss of the most volatile compounds which escape with the discharged solvent vapours.
The Italian patent application No. Ml 2000A001634 dated Jul. 19, 2000 and the corresponding European patent application no. 01114900.2, the content of which must be considered incorporated herein for reference, describe an injection device and a method of vaporization injection, in which the sample (formed of the substance to analyse and a related solvent) is introduced into the hot vaporization chamber at a high speed, so that it travels the entire length of the chamber in the form of a liquid bond. This band then hits a stopping and vaporization means, such as an obstacle, a packing or similar, at the bottom end of the chamber, where the liquid vaporizes to allow the introduction of the sample in the form of vapour onto the gas chromatographic column, the inlet to which is directly adjacent.
This technique makes it possible to avoid the problems as described in the cited application, of vaporization of the sample in the needle of the introduction syringe, and at the same time allows the length of the vaporization chamber to be extended to over 80 mm and hence samples of relatively large volume to be injected without problems of losses caused by overflow through the head of the chamber and/or the septum purging duct (see EP 699 303 for injection with overflow), and naturally without the loss of volatile substances which may occur if the solvent is eliminated before entering the column.
Nonetheless, although there is a considerable increase in the injectable volumes, from the 1-3 μl of conventional injectors to about 5-10 μl, these volumes are necessarily limited by the volume of the chamber, which cannot be increased as desired.
This being stated, the main object of the present invention is to provide a method and a device for vaporization injection in splitless mode of even larger sample volumes, without elimination of the solvent vapours towards the outside, that is by introducing the entire sample onto the gas chromatographic column and without phenomena of overflow or vaporization in the injection needle.
To attain this object it is necessary to perform a rapid injection of the sample, so as to create a liquid band of the same inside the heated vaporization chamber. This eliminates the drawbacks caused by the heating of the syringe needle. The problem of preventing the large volume of the vaporized sample, which cannot be contained in the chamber, from being dispersed, even only partly, in the head of the chamber and in septum purging duct (overflow) are still to be solved.
Therefore, the vapours must be sent to the gas chromatographic column at a substantially the same speed as the speed at which they form.
A solution to this problem was proposed, among others, in the publication of Watanabe and Hashimoto (Journal of High Resolution Chromatography Vol. 13 September 1990, 610-613) by using an injector for packed columns, without septum purging duct and without splitting, and injecting large volumes of sample (50 μl), at a low speed (about 5 μl/s) and with an increased speed of the carrier to deliver the vapours to a capillary kept cold during injection. The publications of Suzuki et al (Journal of AOAC International Vo. 77 No. 6, 1994. 1647-1641 and Journal of Chromatography A. 662 (1994) 139-146) also propose the use of a cold trap downstream of the injector, but again with a slow injection (2.5 μl/s) which produces the known problems of vaporization with loss of heavy products which remain in the needle and cause important errors in the analysis of the sample. The cold trap recondenses only the substances to be analysed, but not the solvent; which is discharged to the outside as vapour through a specific duct controlled by a valve.
Elimination of the solvent with the method described in the cited works also produces a loss of light components and, moreover, a deformation of the chromatography peaks, making a correct quantitative evaluation of the compounds with medium or high volatility impossible.
It must also be mentioned that Watanabe and Suzuki inject at controlled speed, which may be attained manually with extreme difficulty, and normally requires a special automatic sampler. Moreover, the appropriate speed must be optimized through experimentation.